1. Field of the Invention
The present invention relates to charge-coupled devices. More specifically, the present invention is directed to radiation sensors utilizing a matrix of charge-coupled devices.
2. Description of the Prior Art
Charge-coupled devices (CCD's) are used to store and transfer packets of electrical charge representing information in sites or charge wells within the CCD's. CCD's are low leakage devices which conserve the injected charge and do not internally generate charge. The input charge can be injected either into a CCD input terminal or into the charge well if it is responsive to input photons. To achieve desired economics, CCD's must be of very high density with each well being relatively small and storing or integrating small amounts of charge. Radiation sensors or imagers containing a matrix of hundreds or thousands of radiation detectors utilizing charge-coupled devices (CCD's) are well-known in the art, e.g., as shown in U.S. Pat. No. 3,876,952, to perform focal plane signal processing functions including signal multiplexing and time-delay-and integration (TDI). One of the problems associated with such focal planes containing a large number of CCD detectors is the variability of gain and DC offset from detector to detector. A prior art method of correcting these non-uniformities is based on the use of a feedback circuit after the signal multiplexer for individually correcting each output signal. Such correction circuits are complex and significantly increase the product cost of the radiation sensor matrix. An additional problem which is encountered n radiation detector focal planes is the large CCD storage wells required to handle the DC charge produced by radiation detector leakage current and background photon flux reaching the detector. In TDI circuits, where the charge from a number of detectors is added to improve the signal-to-noise ratio, this DC charge problem can impose a severe limitation on the number of TDI stages which can be cascaded. A prior art circuit for removing the DC or background representing charge components of a signal with respect to a charge-coupled device in a shift register is shown in U.S. Pat. No. 3,969,634. While this circuit subtracts a DC charge to provide a background subtraction it does not individually compensate for variations of each input circuit. Thus, that prior art approach does not recognize individual variations of the separate input circuits and, accordingly, does not maximize the output from each radiation sensor. Further, in order to provide a suitable background subtraction circuit it must be applicable to both monolithic and hybrid focal plane sensors and to both source and gate coupled inputs for photoconductive and photovoltaic radiation detectors. Additionally, the area of the detector and its associated CCD input circuit should be as small as possible in order to achieve the highest performance or resolution of the radiation sensor to maximize its application potential.